Odd javascript array behaviour. Can it be fixed?

Question

I have a multi-dimensional array like so:

var map = [[0,0,0,0,0],[0,0,0,0,0],[0,0,0,0,0],[0,0,0,0,0],[0,0,0,0,0]];

I then have a few functions that "resize" the array in both dimensions, depending on some variables. This is how I do it:

function resizeArr(arr, rows, rowsDiff, cols, colsDiff) {
    var arrLength = arr.length;

    if (colsDiff > 0) {
        i=0;
        while (i<arrLength) {
            j=0;
            while (j<colsDiff) {
                arr[i].push(0);
                j++
            }
            i++
        }
    }
    if (colsDiff < 0) {
        i=0;
        while (i<arrLength) {
            j=0;
            colsDiffAbs = Math.abs(colsDiff);
            while (j<colsDiffAbs) {
                arr[i].pop();
                j++
            }
            i++
        }
    }

    if (rowsDiff > 0) {
        fullColsArr = makeArrayOf(0, cols);
        i=0;
        while (i<rowsDiff) {
            arr.push(fullColsArr);
            i++
        }
    }
    if (rowsDiff < 0) {
        rowsDiffAbs = Math.abs(rowsDiff);
        i=0;
        while (i<rowsDiffAbs) {
            arr.pop();
            i++
        }
    }
    return arr;
}

Basically, with those 4 if statements, I pop out some values or push some new ones (zeros) depending on whether the rowsDiff/colsDiff variables are positive or negative.

My problem is when I resize the array from a max of map[4][4] to map[9][9], it looks like everything's okay, because I get:

[[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0]]

as my new array, which looks right. However, when I try to assign a particular value to map[9][9], it fills up the last value of each new array like so:

[[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0,14],[0,0,0,0,0,0,0,0,0,14],[0,0,0,0,0,0,0,0,0,14],[0,0,0,0,0,0,0,0,0,14],[0,0,0,0,0,0,0,0,0,14]]

If I try to assign a value to the second to last value (map[9][8]), it fills up the second to last values of all the others, too.

But when I assign a value to an original part of the array, it works properly. Just what am I doing wrong here?

I've tried to explain it to the best of my ability- feel free to ask for further details.

lc. · Accepted Answer · 2012-07-14 21:20:35Z

4

You're pushing the same array multiple times when you expand by rows. Therefore each new row of the array is just pointing the same place (the same column array). Thus when you change one, they "all" appear to change.

Instead, you want to create a new array for each new row and push that:

if (rowsDiff > 0) {
    i=0;
    while (i<rowsDiff) {
        arr.push(makeArrayOf(0, cols));
        i++
    }
}

answered Jul 14, 2012 at 21:20

lc.

117k21 gold badges163 silver badges190 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

DavidPM Over a year ago

Perfect! Thank you :) I don't know why I didn't think of that. Another learning experience for me, hehe.

Collectives™ on Stack Overflow

Odd javascript array behaviour. Can it be fixed?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related